This invention was made with government support under Grant (Contract) No. DE-AC03-76SF00098 awarded by The U.S. Department of Energy. The government has certain rights to this invention.
This invention relates generally to fume hoods, and in particular to energy-efficient laboratory fume hoods. More specifically, the invention relates to laboratory fume hoods having air supplied through sources at the hood's face.
A fume hood may be generally described as a ventilated enclosed workspace intended to capture, contain, and exhaust fumes, vapors, and particulate matter generated inside the enclosure. The purpose of a fume hood is to draw fumes and other airborne matter generated within a work chamber away from a worker, so that inhalation of contaminants is minimized. The concentration of contaminants to which a worker is exposed should be kept as low as possible and should never exceed a safety threshold limit value. Such safety thresholds and other factors relating to testing and performance of laboratory fume hoods are prescribed by government and industry standards by organizations, such as the American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (ASHRAE) of Atlanta, Ga., for example, ANSI/ASHRAE 110-1995. ASHRAE Standard, "Method of Testing Performance of Laboratory Fume Hoods." This and all other documents cited in this application are incorporated herein by reference for all purposes.
FIG. 1 shows a cross-sectional side view of a conventional fume hood. The hood 100 includes a work chamber 102, bounded by walls 103 and a front open face 105 which may be covered partially or completely by a moveable sash 114. The hood may be supported by a base 104. In many designs, the base contains cabinets for storage of solvents and other materials used in the hood's work chamber 102.
While hood sizes vary considerably, a typical conventional fume hood is about 4 to 8 feet wide with a sash opening of between about 26 and 31 inches, and a standard interior vertical size of about 48 inches. The hood's walls 103 typically have considerable width because they provide an aerodynamically shaped entrance to the work chamber 102 and contain mechanical and electrical services for the hood. Again, while dimensions of fume hoods greatly vary, the depth of a typical fume hood ranges from about 32 to about 37 inches. A typical conventional hood design includes an air foil 106 at the bottom front of the work chamber 102 and a baffle 108 at the rear of the work chamber 102. The depth of the work chamber 102 between these two features 106 and 108 is typically approximately 21 inches.
The air foil 106 at the entrance to the work chamber 102 is an important aerodynamic design feature of the fume hood 100. The air foil 106 is designed to prevent the formation of turbulent air flow in the lower part of the hood's work chamber 102. In a conventional design, the air foil 106 runs at an upward angle from the front plane of the fume hood 110 towards the rear of the fume hood 112.
The opening in the front of the fume hood 100 which provides access to the work chamber 102 by a worker, is referred to as the face of the fume hood. In some conventional fume hood designs, referred to as open-faced hoods, the face area of the hood is fixed. In other designs, such as that depicted in FIG. 1, a moveable sash 114 provides the ability to alter the face area of the hood 100. Sashes come in either vertical or horizontal arrangements, with the vertical design typically being preferred since it can provide a full open face area.
Other elements of conventional fume hoods illustrated in FIG. 1 include an air bypass area 116 above the sash in the top front of the fume hood 100 which provides an additional path for ambient air to enter the work chamber 102. The bypass 116 provides sufficient air flow to dilute contaminants in the hood, and to avoid air whistling when the sash 114 is closed. Air is exhausted from the fume hood through an exhaust system equipped with a fan (not shown) which draws air into the fume hood's work chamber 102, through the baffle 108, and into ducting 118 outside the work chamber 102 of the fume hood 100 for exhaustion from the building. The top wall of the fume hood is also typically equipped with a light fixture 120 to illuminate the work chamber 102. The back baffle 108 typically includes two or three horizontally disposed slots to direct air flow within the work chamber 102. Further details regarding the design and construction of conventional laboratory fume hoods may be found in Sanders G. T., 1993. Laboratory Fume Hoods, A User's Manual. John Wiley & Sons, Inc.
Containment of contaminants in a conventional fume hood is based on the principal of a directed (inward) air flow in the face of the hood. As noted above, the face corresponds to the area below the sash (in the case of a vertical sash arrangement) at the front of the hood through which air enters the work chamber. In a conventional fume hood design, the lower boundary of the face is defined by an air foil, as discussed above.
For safe fume hood operation, the laboratory in which the fume hood is located should be well-ventilated. For typical laboratory operations, six air changes per hour (acph) of outside air are recommended for a safe B-2 occupancy laboratory. Bell, et al., 1996. A design for Energy Efficient Research Laboratories. Lawrence Berkeley National Laboratory Publication No. 777. For laboratories that routinely use hazardous materials, such as carcinogens, ten to twelve outside acph are often recommended.
An important factor in a conventional fume hood's ability to contain contaminants is its face velocity. The face velocity of a fume hood is determined by its exhaust and its open face area. Recommendations for face velocity of conventional fume hoods range from 75 feet per minute (fpm) for materials of low toxicity (Class C: TLV&gt;500 ppm) to 130 fpm for extremely toxic or hazardous materials (Class A: TLV&lt;10 ppm). Cooper, E. C., 1994. Laboratory Design Handbook, CRC Press. In general, industrial hygienists recommend face velocities of 100 fpm for containment of contaminants by conventional hoods with open sashes.
In addition to the hood design, the position of the worker with respect to the air flow direction may have a significant influence on the air flow patterns in the hood, and particularly in the face of the hood. Air flows surrounding a body standing in front of the hood create a region of low pressure downstream of the body. This region, which is deficient in momentum, is called the wake. It disturbs the directed air flow in the face of the hood causing turbulence which may result in reversal of flow causing contaminants to spill from the hood's work chamber into the surrounding laboratory space.
It has also been found that hood leakage is dependent on laboratory air flow patterns. National Institute of Health, 1997. Methodology for Optimization of Laboratory Hood Containment. Volumes 1 and 2. The turbulent fluctuation in air velocity generated in the room surrounding the hood face is carried into the hood by the general flow of air. Therefore, a hood's performance may be affected by the hood's location with respect to doors, supply air outlets and areas with foot traffic.
FIG. 2 shows a cross-sectional side view of a conventional fume hood design, such as that illustrated in FIG. 1, further illustrating ideal air flow through such a conventional hood. Air is shown entering the hood 200 from the surrounding laboratory space 201 by arrows 202. The air flows through the open face 203 of the hood 200 defined by the fully open sash 206 and the air foil 208 into the work chamber 205. Inside the work chamber 205 the air is drawn towards slots 204 in the baffle 207 at the rear of the work chamber 205. In the particular design depicted in FIG. 2, the air flow generated by the slots establishes a vortex 210 in the upper region of the work chamber. If this vortex extends to or below the upper limit of the open face 203, the risk of spillage of airborne contaminants from the hood 200 is increased. Having passed through the baffle 207, the air is then exhausted through the exhaust system 212.
As described above, the air source for conventional fume hoods is the ambient air in a laboratory in which the fume hood is located. The additional air which must be provided to a laboratory space by a building's HVAC system to replace air exhausted by a fume hood is referred to as "make-up air." Since make-up air is supplied as part of the laboratory's ambient air, it must be conditioned to the same degree if comfort and safety levels in the laboratory are to be maintained. As a result, laboratory buildings have very high energy intensities. Conditioning of the make-up air to be exhausted by fume hoods uses most of the energy beyond what is required for technical apparatus and lighting in laboratory environments. The high energy consumption caused by fume hood exhaust air flows is a result of both the need to condition make-up air and in conventional systems and to move it through a building's air flow handling system. Thus, the operation of conventional laboratory fume hoods results in a tremendous energy wastage.
Several attempts have been made to reduce the energy consumption of laboratory fume hoods. In order to maintain an appropriate level of safety, it is not practical to reduce the volume of air exhausted by a conventional fume hood. As noted above, in order to maintain an appropriate safety margin face velocities should be maintained at approximately 100 fpm. Two alternate fume hood designs developed to provide energy savings over conventional fume hood designs are discussed below. The descriptions of these alternate designs use terms described with reference to FIG. 1, and reference to that figure may assist in an understanding of these designs.
A first attempt to save conditioning energy is the auxiliary air fume hood. Auxiliary air fume hoods supply unconditioned (or less-conditioned) air near the top and front of the hood sash outside the front plane of the hood. Therefore, the amount of conditioned room air drawn into and exhausted by the hood is reduced. However, the un/less-conditioned air, which may be up to 95% of the exhaust, often causes thermal discomfort in winter when outside air is cold or in summer when outdoor humidity and temperature levels are high. Auxiliary air can also adversely impact experiments, since the air temperature in the hood's work chamber will not be the same as the ambient laboratory room air temperature. In addition to these problems related to the thermal condition of auxiliary air, the system presents some engineering challenges in providing an air supply of an appropriate volume and velocity to the face area of the hood. Further, while auxiliary air fume hoods reduce the amount of energy used to condition make-up air, and reduce infrastructure costs by permitting installation of downsized heating and cooling equipment, they do not reduce fan energy consumption because they do not change the amount of air exhausted from the hood.
Another alternative fume hood design, referred to as a variable air volume (VAV) hood makes use of the energy saving strategy of controlling the amount of air flow through the hood as a function of the hood's sash location. Conventional constant-volume fume hoods are not constant face-velocity hoods, since the exhaust air fan removes approximately the same amount of air regardless of the sash position. In a vertical sash implementation, if the sash is lowered, the face velocity increases and may reach unsafe levels. For example, it has been found that face velocities higher than 125 fpm can create significant turbulence inside the hood, causing the fumes to spill into the laboratory. Monsen, R. R., 1987. Practical Solutions to Retrofitting Existing Fume Hoods and Laboratories. ASHRAE Transactions. 845-51.
VAV fume hoods are constant face-velocity fume hoods. They are equipped with a variable air volume exhaust fan and automatic controls. Fume hoods equipped with VAV regulate the amount of exhaust from the hood to obtain a relatively constant face velocity. The exhaust air flow can be controlled by sensing the face velocity, the sash position, or the pressure between the inside of the hood and the room outside the hood. VAV systems also control the amount of make-up air by means of multiple dampers. An example of a VAV fume hood system is described by Maust, et al., 1987. Laboratory Fume Hood Systems, their use and Energy Conservation. ASHRAE Transactions. 1813-19.
VAV fume hoods are theoretically safer than conventional hoods, because the face velocity stays constant independent of the sash position. In addition, if the sash is less than fully open for a significant period of time, a VAV system may result in significant energy savings. However, user discipline, or automatic controls to determine whether a person is present at the hood, are necessary for the VAV system to save energy. A further disadvantage of the VAV system is the relative complexity of the automatic systems which must be in place for such a system to function.
Accordingly, alternative low energy consumption fume hood designs would be desirable.